As is well known, beam-indexing color CRTs have a phosphor screen comprising a multiplicity of phosphor index stripes arranged on the inner surface of a panel and three primary phosphor color stripes repeatedly arranged on the panel inner surface in a definite relation with the phosphor index stripes. When the phosphor screen is scanned by a single electron beam, an index light signal is obtained, which is utilized for the electron beam to excite the desired phosphor color with a specified amount of electron beam for the reproduction of color images.
There are two types of flat, color CRTs incorporating such a beam-indexing system. FIG. 5 shows the structure of one of these types. With reference to FIG. 5, a flat glass tube 1 comprises a neck 3 accommodating an electron gun 2, a funnel 4 and a panel 6 provided with a phosphor screen 5 (shown in FIG. 6) on its inner surface. The phosphor screen 5 is inclined with respect to the central axis of the electron gun 2 (to the direction of propagation of an electron beam 7 when the beam is not deflected).
The phosphor screen 5 comprises red, green, and blue, i.e., three primary phosphor color stripes 8 repeatedly arranged on the inner surface of the panel 6. A metallic layer 9 of aluminum is formed over the resulting pannel inner surface. Phosphor index stripes 10 are provided on the metallic layer 9 in a definite relation with the primary phosphor color stripes 8 (FIG. 6).
With such a flat CRT, the single electron beam 7 emitted by the electron gun 2 is deflected by a deflection yoke 14 and scans the phosphor screen 5 to produce index light, which strikes a light collector plate 11 disposed on the rear side of the funnel 4. The index light collected by the plate 11 is converted to a wavelength which matches the sensitivity of a photodetector. The light is led from the plate 11 to a photodiode 12 provided at one end of the light collector plate 11. The photodiode 12 produces an electric signal upon conversion. Published Unexamined Japanese Patent Application No. SHO 57-65651 discloses a light collector plate which is usable as the plate 11.
On the other hand, the electron beam 7 passes through the metallic layer 9 and excites a phosphor color stripe 8, whereupon the stripe luminesces. The luminescence is observed through the panel 6.
However, the flat, color CRT of the above construction has a drawback in that the color image reproduced is insufficient in luminance because the electron beam 7 excites the phosphor color stripe 8 through the metallic layer 9. Since the phosphor screen 5 is arranged as inclined with respect to the axis of the electron gun 2, the beam 7 is incident on the screen 5 obliquely. This results in the electron beam passing through the metallic layer 9 traveling a longer distance. An increased proportion of the electron beam energy therefore attenuates within the metallic layer 9 to further reduce the luminance of luminescence of the phosphor color stripe 8.
Published Unexamined Japanese Patent Application No. SHO 57-27541 discloses a flat, color CRT of the other type which is adapted to overcome the above drawback. FIGS. 7 and 8 schematically show the construction of the CRT. With reference to FIG. 7, a phosphor screen 50 comprises phosphor index stripes 10 provided on the inner surface of a panel 6, a metallic layer 9 of uniform thickness formed over the panel inner surface to cover the stripes 10, and primary phosphor color triplet stripes 8 provided on the metallic layer 9. In this case also, the phosphor index stripes 10 are of course arranged in a definite relation with the arrangement of phosphor color stripes 8.
With the flat, color CRT of this structure, an electron beam 7 directly excites the phosphor color stripe 8 for luminescence, and the luminescence is reflected from the metallic layer 9 toward the interior vacuum space of the flat glass tube 1, so that a bright color image can be observed through a window formed in a funnel 4.
Nevertheless, the phosphor screen 50 has a drawback. With reference to FIG. 8 showing the screen 50 in greater detail, the electron beam 7 passes through the metallic layer 9 and excites the phosphor index stripe 10, which therefore produces weak luminescence. Consequently, the index light incident on a light collector plate 11 through a panel 6 is low in intensity.
In the case of beam-indexing color TV receivers, it is necessary to obtain an index signal at all times in order to detect the position of the electron beam as is well known, so that even for the reproduction of a black image, a beam current of not lower than a specified level is passed. Accordingly, it is desirable that a smaller amount of electron beam is needed for the index signal for reproducing of a black level to give improved contrast to the image. However, because the metallic layer attenuates the energy of the electron beam as mentioned above, it is required for reproducing a black level, the amount of electron beam be larger in the CRT of the second type than where the electron beam directly excites the phosphor index stripe. This invariably results in lower contrast.
To overcome this drawback, we, the applicants, have already proposed a flat CRT of the following construction in Japanese Patent Application No. SHO 59-77772 (filed on Apr. 17, 1984 and published Nov. 5, 1985.)
FIG. 9 shows a sectional view of the phosphor screen of the proposed CRT. Three primary phosphor color stripes R (red), G (green) and B (blue), are arranged at a predetermined spacing, are formed on an aluminum metallic layer 16 on the inner surface of a panel 6. On the other hand, phosphor index stripes 17 are provided on the inner surface of the panel 6 and positioned in spaces 18 between the phosphor color stripes R, G, B, as arranged in a definite relation with these color stripes. No metallic layer 16 is formed on the phosphor index stripes 17. The metallic layer 16 in contact with the phosphor color stripes R, G, B has a thickness L.sub.1 which is sufficiently large so that when the electron beam excites the phosphor color stripes R, G, B, the resulting luminescence is totally reflected from the metallic layer 16 without passing therethrough.
With the construction described above, the luminescence of the primary phosphor color stripes R, G, B by the electron beam 7 is totally reflected at the metallic layer 16 and released toward an observation window 13, enabling the viewer to observe a bright color image through the window 13. Moreover, with no metallic layer 16 formed over the phosphor index stripes 17, the electron beam excites the phosphor index stripe 17 without attenuation. This results in an index light of high intensity available at the light collector plate through the panel 6. When necessary, an electrically conductive transparent film 19 can be provided for the observation window 13 of the funnel 4. The film 19 is maintained at the same potential (anode potential) as the metallic layer 16.
For the flat CRT to produce images with still improved contrast, a nonluminescent substance such as carbon must be interposed between the primary phosphor color stripes. The nonluminescent substance commercially available generally comprises a mixture of carbon and an aqueous solution of ammonia or like alkali material so as to render the carbon effectively separable. However, due to the presence of the aqueous solution, the nonluminescent substance is not compatible with the metallic film of aluminum. Therefore, it is extremely difficult to form carbon stripes on the metallic layer 16 using the nonluminescent substance.